January 23, 2017 5:21 AM EST

The weather may, in fact, be a harbinger of health trouble.

The age-old claim that drastic changes in temperature can spur sickness seems to be based, at least in part, on science. In a recent study, Swedish researchers analyzed more than 20,000 nasal swabs from people who’d gone to the doctor feeling under the weather and found that, indeed, flu outbreaks first appear about a week after winter’s first cold spell. Why? Airborne particles containing flu virus can spread more easily in cold and dry weather, the study authors wrote.

Wearables can detect changes that signal an oncoming cold.

Stanford University School of Medicine researchers gave 60 people health trackers (like the iHealth Pulse below) and analyzed 2 billion pieces of data they captured, including heart rate, sleep, fitness and blood-oxygen levels. They noticed that subjects’ vitals changed shortly before they got sick, suggesting that wearables may one day be able to signal such drop in your health status–before your symptoms get bad.

This appears in the January 30, 2017 issue of TIME.

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